Optical coherence analysis relies on the use of the interference phenomena between a reference wave and an experimental wave or between two parts of an experimental wave to measure distances and thicknesses, and calculate indices of refraction of a sample. Optical Coherence Tomography (OCT) is one example technology that is used to perform usually high-resolution cross sectional imaging. It is often applied to imaging biological tissue structures, for example, on microscopic scales in real time. Optical waves are reflected from an object or sample and a computer produces images of cross sections of the object by using information on how the waves are changed upon reflection.
The original OCT imaging technique was time-domain OCT (TD-OCT), which used a movable reference mirror in a Michelson interferometer arrangement. More recently Fourier domain OCT (FD-OCT) has been developed. Two related FD-OCT techniques are time encoded and spectrum encoded OCT. These Fourier domain techniques use either a wavelength swept source and a single detector, sometimes referred to as time-encoded FD-OCT (TEFD-OCT) or swept source OCT, or, alternatively, a broadband source and spectrally resolving detector system, sometimes referred to as spectrum-encoded FD-OCT or SEFD-OCT. These three OCT techniques parallel the three spectroscopy approaches implemented by Fourier transform spectrometers, tunable laser spectrometers, and dispersive grating with detector array spectrometers.
These various OCT techniques offer different performance characteristics. FD-OCT has advantages over time domain OCT (TD-OCT) in speed and signal-to-noise ratio (SNR). Of the two Fourier Domain OCT techniques, swept-source OCT or TEFD-OCT has distinct advantages over SEFD-OCT because of its capability of balanced and polarization diversity detection; it has advantages as well for imaging in wavelength regions where inexpensive and fast detector arrays are not available.
TEFD-OCT or swept source OCT has advantages over SEFD-OCT in some additional respects. The spectral components are not encoded by spatial separation, which requires bulky grating arrangements, but they are encoded in time, which can utilize compact swept wavelength sources. The spectrum is either filtered or generated in successive frequency steps of the swept source and is reconstructed before Fourier-transformation. Using the frequency scanning swept source the optical configuration becomes less complex and more compact, but the critical performance characteristics now reside in the source and especially its tuning speed and accuracy.
The swept sources for TEFD-OCT systems have been typically tunable lasers. The advantages of tunable lasers include high spectral brightness and relatively simple optical designs. The typical tunable laser is constructed from a gain medium, such as a semiconductor optical amplifier (SOA), placed inside an optical laser cavity which includes an intracavity tunable filter, such as a rotating grating, fixed grating with a rotating mirror, or a Fabry-Perot tunable filter. Currently, some of the highest speed TEFD-OCT lasers are based on the laser designs described in U.S. Pat. No. 7,415,049 B1, entitled “Laser with Tilted Multi Spatial Mode Resonator Tuning Element”, by D. Flanders, M. Kuznetsov and W. Atia. These highly integrated designs allow for a short laser cavity that keeps the round-trip optical travel times within the laser cavity short, so that the laser is fundamentally capable of high speed tuning Secondly, the use of micro-electro-mechanical system (MEMS) Fabry-Perot tunable filters combines the capability for wide spectral scan bands with the low-mass high mechanical resonant frequency deflectable MEMS membranes that can be tuned rapidly.
Another swept laser source for OCT is the Frequency Domain Modelocked Laser (FDML) as described in U.S. Pat. No. 7,414,779 B2. FDML lasers use semiconductor optical amplifiers in a very long, kilometer or more, fiber ring laser cavities that require polarization control and active length stabilization.
The use of laser-based swept sources, however, does have problems. The instantaneous laser emission is characterized by one or more longitudinal laser cavity modes that simultaneously lase within the passband of the laser's tunable filter. Then as the laser tunes, the power within these modes shifts between the modes and to new cavity modes that see gain as the tunable filter passband shifts. This multi-mode spectral structure of the laser emission increases relative intensity noise (RIN), which degrades performance of OCT systems. Another problem is that tunable lasers using ubiquitous semiconductor gain media generally only tune well in one direction, i.e., to longer wavelengths. This is due to a nonlinear asymmetric gain effect in semiconductors that is often called the Bogatov effect. With an optical signal in a semiconductor at a given wavelength, optical waves at longer wavelengths will experience slightly higher optical gain, while optical waves at shorter wavelengths will experience slightly lower optical gain. Such asymmetric nonlinear gain distribution creates a preference for dynamic tuning in the longer wavelength direction, where optical gain is slightly higher, while impeding tuning in the shorter wavelength direction.
Another limitation of tunable laser sources is that their tuning speed is limited by the round-trip time of the laser cavity. Shortening the laser cavity allows for faster scan speeds, but increases the longitudinal mode spacing and thus reduces the number of modes that can lase within the filter linewidth. The reduced number of lasing mode increases the RIN, and can ultimately lead to mode-hopping. On the other hand, one can increase the filter linewidth to allow a larger number of modes to lase for a lower laser RIN, but this increased laser linewidth results in shorter coherence length that may not be adequate for imaging deeper objects. Potential maximum imaging depth of a swept source OCT system is given by one half the coherence length of the system source, where the coherence length is inversely proportional to the dynamic linewidth of the swept source. Moreover, for a given cavity length and filter linewidth, increasing scan speeds will reduce coherence length and ultimately cause the source to cease lasing.
Another class of swept sources that have the potential to avoid the inherent drawbacks of tunable lasers is filtered amplified spontaneous emission (ASE) sources that combine a broadband light source, typically a source that generates light by ASE, followed by tunable filters and optical amplifiers. Some of the highest speed devices based on this configuration are described in U.S. Pat. No. 7,061,618 B2, entitled Integrated Spectroscopy System, by W. Atia, D. Flanders P. Kotidis, and M. Kuznetsov, which describes spectroscopy engines for diffuse reflectance spectroscopy and other spectroscopic applications such as OCT. A number of variants of the filtered ASE swept source are described including amplified versions and versions with tracking filters.
More recently Eigenwillig, et al. have proposed a variant configuration of the filtered ASE source in an article entitled “Wavelength swept ASE source”, Conference Title: Optical Coherence Tomography and Coherence Techniques IV, Munich, Germany, Proc. SPIE 7372, 737200 (Jul. 13, 2009). The article describes a source with an SOA functioning both as an ASE source and first amplification stage. Two Fabry-Perot tunable filters are used in a primary-tracking filter arrangement, which are followed by a second SOA amplification stage.
Such swept filtered ASE sources, while typically more complex optically than some lasers, do provide some performance advantages. For example, they do not have laser optical cavities and thus do not have the laser tuning speed limitations imposed by the finite cavity roundtrip time. Moreover, the lack of the laser cavity avoids the problems associated with the discrete longitudinal laser cavity modes.